We propose that nonhuman primates and human children rely on the same basic cognitive processes for responding to the numerical properties of stimulus sets. Children aged 3? to 5 years, chimpanzees, monkeys, and human adults are hypothesized to represent numerosity approximately rather than exactly. We will determine whether children and non-human primates reliably select the largest of multiple sets of items when those items are presented one-by-one into opaque containers with trial durations ranging from 10 minutes to 8 hours. We will determine whether numerical or non-numerical concurrent cognitive tasks performed during these extended-duration trials interfere with performance in these numerousness judgments. We predict that, but will test whether, children and nonhuman primates will respond on these tasks to numerosity rather than dimensions such as amount or surface area. We will determine whether adult humans who are prevented from using formal counting procedures show performance levels comparable to those for children and nonhuman primates when estimating set sizes. We predict that they will. We will determine whether chimpanzees and rhesus monkeys can enumerate a given number of responses when presented with a symbol for numerosity and whether this process of enumeration shows similar attributes to that for making numerousness judgments. We predict that these species will enumerate sets through use of an estimation process similar to that used for making numerousness judgments. The data from these studies will be informative in understanding the role of numerical estimation in these groups as well as the potential mechanisms underlying such estimation: The comparative analyses will illustrate whether numerical representation reflects cognitive continuity across these species. The data from studies with children will provide insight into difficulties children may experience with formal arithmetic instruction